1. Field of the Invention
The present invention relates generally to a method and device for reducing or eliminating the axial thrust commonly associated with rotary machines such as centrifugal, axial, turbo- and other rotary pumps, compressors, motors, pneumatic and hydraulic turbines, turbine engines and other similar machines. More specifically, the present invention relates to rotary machines having a sub-dividing disk located in the cavity between the rotor and the housing for changing the nature of pressure distribution along the outside of the rotor and reduce the dependency of the axial thrust on the wear state of the rotary machine seals.
2. Description of the Prior Art
Rotary machines are widely used in various industries. Centrifugal compressors and pumps, turbo-, gas-, and jet engines and pumps, axial flow pumps and hydraulic motors are just some examples of rotary machines. A typical single- or multi-staged rotary pump or compressor contains a rotor surrounded by a stationary shroud or housing. An active part of the rotor is sometimes also called an impeller which typically contains an arrangement of vanes, disks or other components forming a pumping element that transforms its kinetic rotational energy to the pumping fluid.
One known feature of practically all rotary machines is the presence of the axial force also known as axial thrust which impacts the performance of the rotor. Depending on the rotational speed, rotor diameter, fluid dynamics, annular gap leakage flows and many other parameters, the axial thrust produced may reach such significant levels and as such present a challenge to the longevity and reliability of the rotary machine operation. Axial load is especially harmful for the axial thrust bearings. Failure of the axial thrust bearing can cause general failure of the machine. Expensive procedures of bearing replacement comprises a significant part of the overall maintenance of the rotary machine, especially turbo-jet engines and similar machines in which access to the axial bearings is quite difficult. The need therefore exists for a device that would reduce or better yet make insignificant axial thrust in a rotary machine in order to improve its reliability and extend the time between repair services.
It is also known in the art of rotary machines that the level of axial thrust forces depends on the wear state of the rotor seals of the machine. As the seals wear out, the annular gap leakage flow increases which changes unfavorably the hydrodynamic nature of the vortex flows in the cavities between the rotor and the housing of the rotary machine and typically causes the increase in the axial thrust. That in turn causes higher loads on the axial thrust bearings and may bring about their premature failure.
The challenge of reducing the axial thrust has been long recognized by the designers of the rotary machines. A variety of concepts has been proposed in the prior art in attempt to solve this problem. One of the most popular methods of reducing the axial thrust is the use of a balancing disk or drum. A balancing drum or disk is added in the back of the rotor and placed in its own balancing cavity in such a way that one side of the disk is subjected to high fluid pressure in order to compensate for the axial thrust cumulatively developed in all the prior stages of the machine. Examples of various designs of such balancing disks can be found in U.S. Pat. No. 5,591,016 by Kubota; U.S. Pat. No. 5,102,295 by Pope; U.S. Pat. No. 4,892,459 by Guelich; as well as U.S. Pat. Nos. 4,538,960 and 4,493,610 by Iino. Although capable of reducing the axial thrust to a certain extent, these devices are not generally capable of eliminating the problem over a wide range of rotor speeds and pumping conditions. In addition, they are not as simple to implement, require their own maintenance service and increase the size, inertia and weight of the rotary machine which ultimately reduces its efficiency of operation. They also increase the annular gap leakage and in addition can not compensate for the increasing axial thrust due to the wear of the rotary machine seals.
Another method of axial thrust compensation is to increase the fluid pressure in the appropriate cavity of the rotary machine to exert higher pressure on the rotor and therefore to compensate for the axial thrust. Various additional fluid passages have been proposed in the rotary machines of the prior art for the purposes of creating conditions of changing the fluid pressure against the certain areas of the rotor. Examples of single- and multi-staged rotary machines utilizing these devices are described in U.S. Pat. No. 5,862,666 by Liu; U.S. Pat. No. 5,358,378 by Holscher; U.S. Pat. No. 5,104,284 by Hustak; and U.S. Pat. No. 4,170,435 by Swearingen. Rotary machines of this type employ complicated monitoring and control devices designed to adjust the leakage rates and the pressure values of the additional fluid passages in order to compensate for the axial thrust over a wide range of operating parameters. In addition to complexity, another limitation of this approach is the hydraulic losses associated with these compensating fluid passages which negatively affect the hydraulic and overall efficiency of the rotary machine. As with balancing disks, these devices require separate maintenance and thus increase the operation costs of the machine.
One of the simplest and quite efficient ways to address the problem of the axial thrust is the use of so called swirl brakes described for example in the U.S. Pat. No. 5,320,482 by Palmer or in the article by J. M. Sivo entitled "The influence of swirl brakes on the rotordynamic forces generated by discharge-to-suction leakage flows in centrifugal pumps" (Transactions of ASME, Volume 117, March 1995, pages 104-108). A plurality of stationary ribs, grooves, cavities or vanes located along the housing of the rotary machine are utilized to change favorably the fluid pressure distribution outside the rotor in order to reduce the axial thrust. Although simple and reliable, this method has its own limitations such as creating additional localized vortexes and areas of hydraulic disturbances in the rotary machine which reduces its hydraulic efficiency.
Finally, another method of axial thrust reduction is proposed in the U.S. Pat. No. 4,867,633 by Gravelle. Hydraulic thrust balance is achieved and continuously maintained according to that patent by the controlled axial movement of the rotor shaft and the rotor in order to modulate the gap at the rear seal and therefore control the pressure acting on the back side of the rotor. In that case, an outward thrust force resulting from the rotor operation counterbalances an inward thrust force resulting from the pressure acting on the front side of the rotor. This device is quite complicated and delicate and requires careful adjustment for proper operation. It also reduces the hydraulic efficiency of the machine.
The need exists therefore for a device to reduce axial thrust that is simple in design, is easy to install in existing rotary machines, does not require monitoring and control devices in order to work properly, and is effective in its function over a wide range of operating parameters of the rotary machine.
The need also exists for a device to reduce and control axial thrust that would allow to reduce or preferably eliminate completely the dependency of the axial thrust forces on the wear state of the seals in a rotary machine.